Services on a network are diversified and new services taking advantage of the network are expanding. As a representative example, there is a merged service of broadcasting and communication, that is, an integration of broadcasting, Internet, and telephone (voice communication) services called triple services. This service is a representative application of existing information services, and triple play has become a keyword indicating the next-generation network concept to accommodate diversified information communication services.
In such circumstances, in access networks, construction of FTTH by PON (Passive Optical Network) becomes the mainstream. A PON system comprises an office side apparatus OLT (Optical Line Terminal) located in the office building of a communication carrier, and a plurality of subscriber connecting apparatus ONTs (Optical Network Terminals) each being located at a user home. Signals are distributed to individual homes in a point-to-multipoint form, by laying a single optical fiber (trunk optical fiber) from the OLT to a service area, diverging the trunk optical fiber into a plurality of branched optical fibers by a splitter and connecting each branched optical fiber to the ONT.
Since the PON system has the function of multicasting signals through optical branching, it is useful, for example, as an infrastructure for distributing large-capacity of data such as high-resolution images. Further, as a plurality of OLTs can share the trunk optical fiber, the PON system has an advantage that the costs of laying optical fibers and the number of transmitting/receiving devices in the OLT side can be reduced in comparison with a star-type connection in which the office building and each user home are connected in a point-to-point manner. Current PON systems include G (Gigabit-capable)-PON of the ITU-T standard (Non-patent Documents 1 to 3) and GE (Gigabit-Ethernet)-PON of the IEEE standard (Non-patent Document 4).
During the expansion of merged services of broadcasting and communication attracting attention, a further increase in communication density (higher-level multiplexing), an increase in communication speed (high bit rate), and expansion of fiber laying area are demanded for PON systems in order to distribute high-resolution images such as, for example, high definition TV to a large number of users. The standardization group (IUT-T and IEEE) related to PON is starting a study of the next-generation PON to be a successor to current PON systems.
Presently, 10GE-PON and WDM-PON are proposed in these standardization conferences as next-generation PON. As a multiplexing method for next-generation PON, time division multiple access (TDMA) is mainstream like the current PON, and application of code division multiple access (CDMA) is being studied as another multiplexing method. CDMA has the advantage that it has higher information transmission efficiency per transmission bandwidth than TDMA, because it enables concurrent transmission/reception of a plurality of flows with the same carrier, and adjustment of communication timing among ONTs and the securement of guard time between frames are unnecessary. Moreover, since CDMA can protect transmission data by spread spectrum with orthogonal spreading code, increasing the secrecy of information can be expected in PON systems that accommodate a large number of users on a single optical fiber.    Non-patent Document 1: ITU-T G.984.1 “Gigabit-capable Passive Optical Networks (GPON): General characteristics”    Non-patent Document 2: ITU-T G.984.2 “Gigabit-capable Passive Optical Networks (GPON): Physical Media Dependent (PMD) layer specification”    Non-patent Document 3: ITU-T G.984.3 “Gigabit-capable Passive Optical Networks (GPON): Transmission convergence layer specification”    Non-patent Document 4: IEEE 802.3ah “CSMA/CD Access Method and Physical Layer Specification Amendment: Media Access Control Parameters, Physical Layers, and Management Parameters for Subscriber Access Networks”